Breastmilk circulated throughout the Atlantic world. It transferred from women to infants. It served as a key ingredient in healing remedies. It featured in paintings of the Nursing Madonna and was collected in vials as a relic. Spanish and indigenous wet nurses sold their milk while slave owners stole it from enslaved African women. Breastmilk thus carried religious, medical, ethnic, social, and cultural valences. It also formed bodily bonds which provoked reactionary anxieties in its various settings. I suggest that these bonds formed through breastmilk intimately informed individuals’ understanding of identity and the formation of familial relationships. My dissertation project asks why breastfeeding created social anxieties among prescriptive writers, why these anxieties did not seem to matter to the women and families involved in feeding infants, and how early modern understandings of the body and its fluids underpinned notions of identities and forged kinship bonds. To answer these questions, I will focus on breastfeeding and wet nursing in Spain and in New Spain to gauge how people understood and mobilized milk kinship in the chaos of the Atlantic world system. My project draws on the methods of a wide range of quite different scholarship to assess shifts in knowledge about the body, social and cultural exchange, and how these informed interpersonal connections and social customs. I set into dialogue scholars from the history of Christianity, medicine and the body, race and ethnicity, and the family by interrogating current understandings of kinship and identity. Under scrutiny, categories such as religion, ethnicity, family, and social status were far more fluid than the historiography currently allows. An examination of breastmilk and breastfeeding in the early modern Spanish Atlantic world reveals how bodies and the fluids that composed them underpinned the construction of families and eroded traditional social hierarchies.